Isolates of Trichinella from Alaska. Canada, and Kenya were compared in laboratory hosts using three criteria: infectivity, interbreeding, and morphology. It was found that the Canadian (north temperate) isolate had the highest infectivity in the four species of hosts used. The Alaskan (arctic) isolate was less infective to white rats and A/J mice than the Kenyan (tropical) isolate, but the infectivity of the arctic isolate to cotton rats and guinea pigs was similar to that of the tropical isolate. Interbreeding experiments, using single pairs of infective larvae in all isolate combinations in A/J mice, have demonstrated a complete reproductive isolation between the arctic and the other two isolates, and it partial reproductive isolation between the north temperate and tropical isolates, such that the tropical females could not be fertilized by north temperate males nor by north temperate – tropical hydrid males. North temperate adults were found to be longer than those of the other isolates, and so were the uteri of north temperate females. Since the numbers of larvae produced could be dependent on the length of the uterus, infectivity could be a function of uterus size. In fact, a direct correlation was established between the uterus lengths of the three isolates and their respective infectivities in A/J mice. It is concluded that the north temperate, tropical, and arctic isolates most probably correspond to the three species of Trichinella, T. spiralis, T. nelsoni, and T. nativa respectively (Britov.V. A. and S. N. Boev. Vest, Akad. Nauk Kaz. SSR, 28: 27–32 (1972)).